The Future : Time Travel

One question that philosophers have puzzled over, along with physicists and scientists, is with regard to time travel - that is, is time travel possible? Science fiction stories are littered with incidents where people travel back and forth in time, whether it be in a tardis or a time machine or some other suitable device.

Of course these stories vary greatly in the thought put into them - some are meant to be possible whilst others are blatantly pure fantasy. In some stories, people are allowed to change the past, to interact with the past, or to alter the future and so on as they travel around.

What is required of a story involving time travel is purely that it is consistent - that it does defy the rules of logic. Therefore one cannot literally change the past - since definitionally the past is what has happened - if it can be changed then in what sense can be truly say that it has happened?

Therefore this would have to involve time branching, though it is unclear whether an appeal to branching time would help much to explain 'changes to the past'. Therefore stories will have to be made consistent by saying that you were always there at that time in the past, though it was your future self that was present at a time.

So, if I build a time machine and go back to watch the dinosaurs, then I was always there at that time in history. Hence I cannot change the past - whatever I did in the past, I always did, just before I was born. This is consistent.

But is time travel actually possible - can we make a consistent story that could happen? This is the big question, and debate rages about it. However, one thing is reasonably uncontentious, and to say what this is we need to distinguish between two forms of time travel. Namely, to the past and to the future.

Many assume that if you can do one then you can do both, and if you can't do one then you can neither. However, most now agree that you can travel to the future, it is with regard to the past that they argue.

So, how do you time travel to the future? Well, there are two methods. The first one many think does not really count as time travel - that is, to slow down. To hibernate, like a hedgehog. It sits there and slows its body processes down greatly, so that when it awakes months later, it has aged very little at all since all the reactions in its body have slowed down.

Many do not think that this counts as time travel. The second form is accepted to count as time travel, though it may surprise some people, but upon reflecting on it, it can be seen to be what must really happen in future time travel.

This method is the opposite to the above - it is to speed things up, or to be exact, to speed your movement up. If you were in a rocket that went at the speed of light and circled the earth, then something extraordinary would happen, due to something called the time dilation effect. Let's say on earth it is the year 2000.

You set off into space in the rocket and circle the earth at the speed of light. When it is 2100 on earth, the rocket is brought back down to earth. Extraordinarily, you will have only aged a few seconds - and will have only experience the passage of this amount of time.

Although it is 2100 on earth, to you it will seem as though you have only been gone for a very short time in indeed, and if you were 20 when you left, you will be 20 to all intents and purposes when you return. You will still be a picture of youth and health, whilst your friends are in their coffins.

This is therefore straightforwardly time travel into the future. You experience the passage of very little time and hence travel into the future. If you think about it, you will see that this is time travel. Is it possible? Absolutely, this is fact.

If we could accelerate people to the speed of light, or close to it, safely, then we could do this experiment. However, on a lesser scale experiments of this sort have been done to prove this effect. If you take two computers, with extremely accurate caesium clocks, accurate to millionths of a second, then you can do an experiment:

You synchronise the clocks, and send one into geo-stationary orbit around the earth. The other you leave on earth. After, say, a year, you return the clock in orbit to earth. You check the times on the two clocks, and see that the one in space has experienced slightly less time, perhaps only a few millionths of a second less, but still less.

This is because it has been travelling much more quickly than the clock left on earth - an example that time travel to the future is possibly by speeding things up.

So, enough of future time travel. The debate of most interest is over time travel to the past, is that possible? People are divided here - many think it is, for others it is not. For it to be possible, it must be possible to produce a consistent story about travel to the past.

Some philosophers think that this is possible - for instance David Lewis. Others think that it is not, in fact this camp harbours perhaps the majority of philosophers.

For a full analysis the concept of time needs to be examined with respect to causation - the linkage and direction of cause and effect. This is because time travel to the past requires that the effect can happen before the cause, whereas in everything we see on earth, the effect comes after the cause: i put pressure on the handle, causing the door to open; the door doesn't open before i put pressure on it.

Therefore the question of past time travel needs reverse causation to be possible. Therefore one needs to examine whether reverse causation is logically possible or not.

This requires analysis beyond the scope of this brief article, however if you are interested in a for and against debate, you could try reading: Two Paradoxes of Time Travel by David Lewis; and Real Time II by Hugh Mellor.

What do you think about time travel - is it possible? If you have an opinion either way, please do let us know.